What is a DNSBL?

The goal of a DNS Blacklist is to prevent spam.

Domain Name System Blacklists, also known as DNSBL’s or DNS Blacklists, are spam blocking lists. They allow a website administrator to block messages from specific systems that have a history of sending spam.

These lists are based on the Internet’s Domain Name System, or DNS. It converts numerical IP addresses such as 140.239.191.10 into domain names like example.net. This makes the lists much easier to read, use, and search. If whoever maintains a DNS Blacklist has received spam from a specific domain name in the past, that server would be “blacklisted.” All messages sent from it would be either flagged or rejected from all sites that use that specific list.

The History of the DNSBL

DNS Blacklists have a rather long history in web terms. The first one was created in 1997. Called the RBL, its purpose was to block spam email and to educate Internet service providers and other websites about spam and its related problems. It had three basic components: A domain name to host it, a server to host that domain, and a list of addresses to publish.

Modern DNS Blacklists are rarely used as educational tools. However, their function as a blocker and filter still serves its original purpose. Almost all email servers today support at least one DNSBL in order to reduce the junk mail clients receive. The three basic components of a DNS Blacklist also haven’t changed since RBL was created.

DNS Blacklists Today

There are now dozens of different DNSBLs available for use. Each one has its own list, populated based on their own standards for what is or isn’t spam. Because of this, DNS Blacklists can vary greatly. Some only list sites for a certain amount of time since the last spam was received. Some are maintained manually. And some don’t just block IP addresses, but entire Internet Service Providers known to harbor spammers. This means some lists works better than others because they are maintained to different standards.

Website admins can use these differences to decide which DNSBL works best for their security needs. Less strict lists might let more spam through, but might not block non-spam messages that have been misidentified by stricter lists. DNSBLs that are intended for public use usually have a published policy detailing their criteria. This way, they not only attain confidence in their services, but sustain it.